1. Field of the Invention
The invention is related to the field of electric wireline formation fluid testing instruments, and to apparatus and methods for characterizing samples of connate fluids withdrawn from earth formations by such formation fluid testing instruments.
2. Description of the Related Art
Electric wireline formation fluid testing instruments are used to withdraw samples of connate fluids from earth formations penetrated by a wellbore. Certain characteristics of the fluid samples can be used to infer the nature of the connate fluids in the formations, particularly whether the fluid samples include petroleum, and the physical properties of the petroleum if it is present in the fluid samples. Formation testing instruments typically include one or more sample tanks to transport some of the connate fluid to the earth's surface where the sample may be characterized in a laboratory. See for example, U.S. Pat. No. 5,473,939 issued to Leder et al which describes one such formation fluid testing instrument.
A particular difficulty associated with fluid sampling using electric wireline instruments known in the art has been determining the extent to which the fluid samples placed in the tank contain connate fluids from the earth formation, and the extent to which the samples contain the liquid phase ("mud filtrate") of a fluid ("drilling mud") used to drill the wellbore. The mud filtrate enters ("invades") the pore spaces of the earth formation proximal to the wellbore due to hydrostatic pressure and therefore frequently contaminates samples of fluid withdrawn from the formation.
Wireline formation testing instruments known in the art include various apparatus to overcome this limitation. Generally, the formation testing instruments include a means for withdrawing fluid from the formation and selectively discharging the fluid to the wellbore, rather than to the sample tanks, until it is determined that the fluid being withdrawn consists substantially of connate fluid. See the Leder et al '939 patent, for example, which describes a so-called "pump-through" capability. While withdrawing the fluid from the formation and pumping the fluid through the instrument, one or more properties of the fluid can be monitored. The point at which the nature of the withdrawn fluid changes from mud filtrate to connate fluid can generally be inferred from changes in the properties being monitored. The monitored properties include dielectric constant and electrical resistivity. For example, U.S. Pat. No. 5,677,631 issued to Reittinger et al describes a waveguide which enables making measurements related to the electrical conductivity and/or dielectric constant of the fluid being withdrawn. If water forms the liquid phase of the drilling mud, changes in the conductivity and/or dielectric constant can be related to changes in the nature of the withdrawn fluid. Using conductivity and/or dielectric constant to characterize the fluid being withdrawn from the formation has several limitations. First, the liquid phase of the drilling mud may be hydrocarbon-based rather than water-based, making characterization difficult if the connate fluid includes oil. Second, the connate fluid may contain substantially no hydrocarbons and may have an electrical conductivity very nearly the same as that of the mud filtrate, making determination of the nature of the fluid sample difficult. Finally, if the fluid sample contains both hydrocarbons and water, measuring electrical conductivity and/or dielectric constant in a relatively small volume waveguide, as is necessary within the confines of a typical electric wireline formation testing instrument, can result in noisy and unstable measurements, making accurate fluid characterization difficult.
Other methods for characterizing fluid samples include determining various relationships between the pressure and the volume of the fluid sample, such as described in U.S. Pat. No. 5,635,631 issued to Yesudas et al. A limitation to using the method described in the Yesudas et al '631 patent is that withdrawing the fluid from the formation must necessarily be stopped while the pressure/volume relationship of the fluid sample is carefully determined. Using this method to determine the point at which the fluid sample consists of connate fluid would therefore be impracticable because of the amount of time needed. Further, if the connate fluid were to consist mainly of water, the method in the Yesudas et al '631 would not readily indicate whether the fluid sample contained mud filtrate, connate fluid or any combination thereof.
Near infrared ("NIR") photospectroscopy has also been used to characterize the fluid being withdrawn from the earth formation. U.S. Pat. No. 4,994,671 issued to Safinya et al describes a system for NIR photospectroscopy of fluid samples to determine their nature. It has proven difficult in practice to maintain the quality of optics necessary to reliably perform NIR photospectroscopy in a wireline formation testing instrument, primarily because of the opacity of typical crude oils. Further, photospectroscopic methods are generally unable to determine the nature of the fluid sample if the fluid sample and the mud filtrate are both water-based.
Carbon-13 nuclear magnetic resonance ("NMR") spectroscopy is used to determine the chemical structure of carbon containing compounds. Carbon-13 NMR spectroscopy measures frequency shifts in the nuclear magnetic resonant frequency of carbon-13 resulting from combination of carbon atoms in chemical compounds having specific structures. Determining chemical structures of carbon compounds using NMR spectroscopy requires an NMR spectrometer having a resolution of about 1 part per million. This degree of resolution would require an instrument structure having a static magnetic field which is more homogeneous than would be practical for use in a well logging instrument.